Conventionally, a tire-pressure monitor system (Tire Pressure Monitoring System: TPMS) which informs a driver of tire-pressure information has been known. While the tire-pressure monitor system comprises a tire-pressure sensor unit (hereafter, will be referred to as a sensor unit) which detects a tire pressure for each wheel, and is configured to transmit tire-pressure information as a radio signal from this sensor unit and receive this radio signal by a tire-pressure information device disposed on a vehicle body side to acquire the tire-pressure information. When judged that the tire pressure has decreased based on the received tire-pressure information, the tire-pressure information device displays that on an annunciator to notify it to a driver.
For instance, a sensor unit disposed in the tire-pressure monitor system proposed in the Patent Document 1 (PTL1) has an acceleration sensor built-in, and extracts a gravitational-acceleration component detected by this acceleration sensor. Since the sensor unit circles around an axle shaft of a wheel, the gravitational-acceleration component detected by the acceleration sensor is sinusoidally rippled in accordance with the rotation of a wheel. The sensor unit detects its own rotation angle (turning position) based on this gravitational-acceleration component, and repeatedly transmits a radio signal at the timing when this rotation angle becomes a predetermined angle. Such a transmission is referred to as a fixed-angle transmission mode.
In this system, the tire-pressure information device distinguishes the wheel location to which the sensor unit that transmitted the radio signal is attached, based on the number of pulse signals which the wheel-speed sensors of respective wheels output at the timing when the radio signal transmitted from the sensor unit is received.
Moreover, the sensor unit switches the transmission timing of the radio signal to a periodical transmission mode with a predetermined time period (for instance, 1 minute), after the time period required for wheel location discrimination has passed. Thereby, the tire-pressure information device can monitor a tire pressure periodically.